I'd put the question to people like holdennut and hbm*rais, both of whom built Aussie lowboys (same design I'm going to use in my cab).
An Aussie lowboy only fits a 20" screen, so they'd be very well placed to offer an opinion of how the vertical games look on horizontal screens of that size.
Unfortunately, I haven't bought my 20" screen yet for my setup, so I'm not quite yet able to tell you. Plus I'm still tossing up whether to get ambitious and make it rotate within my cab... but, I'm curious to know what others think, in case mine also gets left in a horizontal orientation permanently.